1. Technical Field
The present invention relates to a Lamb wave type high frequency device and a method thereof. In particular, it relates to a Lamb wave type high frequency device in which a piezoelectric substrate having an IDT electrode is connected with a reinforced substrate and a method of manufacturing it.
2. Related Art
As high frequency resonators, surface acoustic wave elements using a Rayleigh wave, a Leaky wave and an SH wave and Lamb wave elements using a bulk wave such as a Lamb wave are conventionally known.
For example, a Rayleigh wave type surface acoustic wave element is known. In the element, an IDT electrode is formed in the Z′-axis direction on the surface of a quartz substrate called as ST cut quartz.
Such element is disclosed in “Analysis of frequency and temperature characteristics of surface acoustic waves by using an infinite element method” written by Shigeo Kanna in pages 37 to 42 of the technical report of IEICE, US 99-20(199-06), in Shingaku Giho.
Further, an SH wave type surface acoustic wave element is also known, (see JP-A-10-233645, page 3 to page 6, FIG. 1). This device propagates a transverse wave in which the propagation direction of a surface acoustic wave is shifted by 90 degrees from a STW cut quartz, namely a ST cut quartz.
Further, a Lamb wave element using a bulk wave (volume wave) propagating with repeating reflection at the upper and lower surface of a piezoelectric substrate instead of a surface acoustic wave is also known. The element is particularly used for higher frequency since its phase velocity is larger compared to that of the surface acoustic wave (see JP-A-2003-258596 and “A substrate for a Lamb wave type surface acoustic wave element” written by Yasuhiko Nakagawa, Masayuki Momose and Shouji Kakio in page 93 to 96 of the 33rd EM symposium 2004.)
Further, in a Lamb wave type high frequency resonator using the Lamb wave element, a Lamb wave is efficiently excited by using an AT cut quartz substrate as a piezoelectric substrate and setting the relationship between the thickness of the quartz substrate H and the wavelength of the Lamb wave λ to be 0<2H/λ≦10.
In the above disclosures (patent documents and non patent documents), however, the thickness of the piezoelectric substrate is from several μm to several tens μm, thereby handling the substrate is difficult since it is easily broken. In particular, as shown in JP-A-2003-258596, the thickness of the piezoelectric substrate has to be several wavelengths in order to excite a Lamb wave, though a Lamb wave type high frequency device can realize higher frequency compared to a case using a surface acoustic wave. Hence, such device is hardly handled and easily broken compared to the surface acoustic device, further, its yield is lowered.